Once you have a working thesis, you should think about what might be said against it. This will help you to refine your thesis, and it will also make you think of the arguments that you'll need to refute later on in your essay. (Every argument has a counterargument. If yours doesn't, then it's not an argument—it may be a fact, or an opinion, but it is not an argument.) Michael Dukakis lost the 1988 presidential election because he failed to campaign vigorously after the Democratic National Convention. This statement is on its way to being a thesis. However, it is too easy to imagine possible counterarguments. For example, a political observer might believe that Dukakis lost because he suffered from a "soft-on-crime" image. If you complicate your thesis by anticipating the counterargument, you'll strengthen your argument, as shown in the sentence below.
While Dukakis' "soft-on-crime" image hurt his chances in the 1988 election, his failure to campaign vigorously after the Democratic National Convention bore a greater responsibility for his defeat.

By using counterarguments in your essay, you anticipate doubts and preempt objections that a skeptical reader might have; it presents you as the kind of person who weighs alternatives before arguing for one, who confronts difficulties instead of sweeping them under the rug, who is more interested in discovering the truth than winning a point.